This invention relates to improved piston ring sealing in squeeze film dampers and, more particularly, to means for improved distribution of fluid pressures in significant regions of piston ring seals in squeeze film dampers.
Squeeze film dampers are employed in gas turbine engines to damp vibratory motion, and generally include a bearing support member, such as the outer annular race of a rolling element bearing, which is fitted into an annular chamber in a bearing housing in which it is permitted to have some limited radial motion. The annular bearing race fits closely in the annular chamber to define a thin annular squeeze film space between the outer circumferential surface of the race and an opposing circumferential chamber wall. Damper fluid, such as oil under pressure, is supplied to the squeeze film space to provide viscous damping and limit radial motion of the shaft and race, which move as a unit.
A pair of axially spaced apart piston ring seals are fitted in grooves in the outer bearing race to engage the opposing chamber wall and seal off the defined squeeze film space therebetween. Each piston ring seated in its groove defines an annular ring space between a radially inward facing surface on the piston ring and a radially outward facing surface of the groove. Fluid pressure in the ring space may be used to pressurize the piston ring against the opposing housing wall to seal the squeeze film space.
Any radial motion of the annular bearing race in the chamber generates a very high fluid pressure in the annular squeeze film space and viscous flow of the oil therein. Such motion also results in movement of the piston rings in their grooves and generates a pressure wave in the ring spaces. During engine operation, rotor shaft imbalance may cause the shaft and race to have some orbiting motion within the housing chamber, thereby creating circumferentially traveling fluid pressure waves with high and low pressure regions in the squeeze film space and the ring space. The squeeze film pressure wave produces a viscous flow which provides beneficial damping of the rotor shaft motion.
In prior designs, the radial dimension, or gap height, of the ring spaces is typically substantially greater than the radial dimension, or gap height, of the squeeze film. Typically, the squeeze film radial height may be between 5-12 mils (0.005 inch-0.012 inch). In known prior designs the ring space radial height is at least two times the squeeze film height. The magnitude of the pressure wave generated in the ring space is therefore much lower than that of the pressure wave in the squeeze film, because for a given motion of the outer race, the percentage volume change of the ring space is smaller than the corresponding percentage volume change of the squeeze film at a given circumferential location. Ideally, the pressure in the ring space should equal the pressure in the squeeze film at any given circumferential location to ensure sealing of the piston ring against the opposing housing wall. Pressure imbalance between the squeeze film and the ring space at a given circumferential location can result in unseating of the piston rings and damper fluid leakage, with attendant loss of damper effectiveness.
Prior designs provide a circumferentially extending vent space for fluid pressure communication between the squeeze film and the ring space. However, applicants believe the vent space diminishes damper effectiveness because it also provides a circumferential flowpath from the high pressure region in the squeeze film pressure wave to the low pressure region in the squeeze film pressure wave, thereby dissipating the magnitude of the pressure wave in the squeeze film. The vent space may also permit excessive axial motion of the piston rings under certain operating conditions, resulting in leakage.